My only concern on the sigma is that it does not allow to add a filter to protect the lens and it has an extra stop.

But that can be a possibility.

thanks

The Canon has a third of a stop improvement in it's range, which in practice isn't any different because you get the same aperture in the crossover ranges.

I personally use my 14mm L II which also doesn't have filter threads at crowded events and have never had a problem by just keeping the lens cap on when not taking picture.

The design is annoying for landscape photographers though. It would be easy for Sigma to make an attachment for a 105mm filter but for the time being filters are an issue. I'm sure you could easily cobble together some cardboard and tape if you were desperate.

I'm the kind of person who would be annoyed with bad pictures than with taping a filter to a lightproof cardboard holder.